Sourpuss runs in Hackney (4)
I believe the answer is:
crab
'sourpuss' is the definition.
(thesaurus)
'runs in hackney' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'in' means one lot of letters goes inside another.
'hackney' becomes 'cab' (I've seen this before).
'r' put inside 'cab' is 'CRAB'.
(Other definitions for crab that I've seen before include "Marine crustacean (seen on 1 across?)" , "Clawed crustacean" , "Seafood (that can be caught by a rower?)" , "The constellation Cancer" , "A crustacean; type of apple" .)